fdt: Allow non-FDT kernels to boot when CONFIG_OF_LIBFDT is defined

fdt: Allow non-FDT kernels to boot when CONFIG_OF_LIBFDT is defined

The boot commands - bootz/bootm mandate a third argument which is the
address to the FDT blob. In cases where this argument is not specified,
boot fails with a message indicating a missing FDT.

This causes non-FDT kernels to fail to boot. This patch allows both FDT
and non-FDT kernels to boot by making the third parameter to the bootm/bootz
optional.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
[trini: Update again for covering appended DTB case after last revert in
this area]
Signed-off-by: Tom Rini <trini@ti.com>